1. Field of the Invention
The present invention relates to inkjet ink compositions comprising at least one azo pigment and at least one synergist.
2. Description of the Related Art
In general, pigments alone are not readily dispersible in liquid vehicles. A variety of techniques have been developed which can provide stable pigment dispersions. For example, dispersants can be added to the pigment to improve its dispersibility in a particular medium. Examples of dispersants for an aqueous medium include water-soluble polymers and surfactants.
Azo pigments are a specific class of colored pigments, generally falling in the yellow, orange, or red color space. These pigments are typically prepared by mixing a diazonium salt (the diazo component) with a compound capable of reacting with a diazonium salt (the azo coupling component). The type and substituents on each component determine the pigments color and brightness.
Dispersions of azo pigments have been prepared using a variety of different techniques, both physical (such as milling) as well as chemical. For example, azo pigment dispersions have been prepared using dispersants having a structural unit similar if not identical to the chemical unit of the colored pigment. For example, GB2356866 discloses a bis-azo compound derived from diazotized (carboxy/sulfo)-anilines and 1,4-bis(acetoacetamido)phenylenes and their use in pigment and ink compositions. These materials were found to be particularly useful in combination with yellow or orange pigments having nearly the identical structure. GB2356634 describes very similar mono-azo compounds for the same use. GB2364322 describes N-(sulfophenyl)-alpha-(2-methoxy-4-nitrophenylazo)acetacetamides for use with monoazo pigments, particularly yellow pigments. Also, in U.S. Pat. No. 6,451,103, water soluble pigment derivatives, which are pigments further substituted with an acidic functional group or salt, are used to prepare aqueous dispersions of that specific pigment. Inkjet inks are also disclosed.
Other methods of controlling the dispersibility of colored pigments are also known. For example, International Patent Application No. WO00/26304 discloses the preparation of a crystal growth inhibitor that controls the amount of crystallinity developed during the process of preparing a colored pigment and prevents recrystallization of dispersed pigments. The crystal growth inhibitor is prepared using a process similar to the known process for preparing the pigment (for example, by the reaction of a diazonium salt with an azo coupling agent), and can also be prepared in situ during the preparation of the colored pigment.
EP 1 316 588 and U.S. Pat. No. 7,300,504 describe methods of preparing dispersions of azo pigments using mixtures of azo couplers or mixtures of diazonium reagents have also been described, including. For example, EP 1 316 588 describes the preparation of a pigment yellow 74 which is obtainable by coupling a diazo component and a coupler component. The coupler component comprises 2-methoxyacetoacetoanilide and an acetoacetoanilide derivative having a defined formula which can be substituted with a COOH or SO3H group. A similar approach using mixtures of diazo components having these substituents is also described. Printing inks are also disclosed.
While these methods produce dispersions of azo pigments, some of these dispersions have been found to be particularly sensitive to various cosolvents and additives used in inkjet ink compositions. Therefore, there remains a need in the industry for methods of preparing inkjet ink compositions comprising azo pigments, particularly those have long term dispersion stability across various types of ink vehicles.